Chromium in the form of Chromate (CrO4—) is a widely employed and highly effective corrosion preventive pigment. It is the de facto standard for corrosion prevention in primers applied to aluminum, particularly aerospace aluminum stock (2024-T3, and certain 7000 series alloys for instance). In addition hexavalent chromate in the form of Strontium Chromate is the benchmark for corrosion prevention in coil coating primers (whether based on Epoxy-Melamine, Polyester or other binder systems) when applied to Zinc, Zinc-Aluminum or similar galvanized or electro-galvanized coil stock.
There has been extensive activity to eliminate hexavalent chromium from these primer systems, which efforts are driven by two related impeti. First, Hexavalent Chromium is toxic, thus efforts to identify a performance equal substitute to Strontium Chromate are easily understood. Secondly, there are directives promoted in Europe that require a limit on Chromium content, such that a manufactured article may more readily be recycled.
For the foregoing reasons there have been numerous academic research projects (Progress in Organic Coatings 47 (2003) 174-182—Buchheit et al.), and an untold number of industry product development exercises devoted to finding a replacement for Strontium Chromate. There is also a substantial patent literature history describing some of these efforts by industry.
In almost every case examined, however, there appears to be one or more flaws in the touted benefits of the inventive compounds. Typically, the claimed chromate-free inhibitor is in fact not demonstrated in a coating system, but rather in a primer alone. This may disguise a performance flaw, e.g., that the inhibitive pigment leads to blistering. Sometimes, the touted composition is offered based on data for a coating system that is applied only to ferrous metal surfaces. Though it may be free of chromium, the anti-corrosive pigment provides insufficient electrochemical over-potential to prevent corrosion of highly reactive metals such as Aluminum or Zinc.
Finally, it is often the case that the benefit of a chrome-free anti-corrosive pigment is made on the basis of unrealistically short accelerated testing, which is deemed unreliable by those skilled in the art. Indeed, many patented offerings, when independently tested, fail to deliver the long duration of outstanding corrosion prevention expected of Strontium Chromate in benchmark accelerated tests.
The present invention overcomes these deficiencies in previous chrome-free anti-corrosive pigments. It provides proof that the inventive compositions are able to deliver performance on a par with Strontium Chromate when evaluated directly alongside the same in realistic coating systems, appropriate for the substrate and end-use application. The present invention in one preferred form does use a ratio of elements in its composition that is previously disclosed, though in an entirely different field of application. It differs from the prior disclosed systems in that this ratio of elements is used to make a highly crystalline Alumino-Silicate which crystallizes in the Sodalite form, and contains as charge balancing interior anions Permanganate (MnO4−). This general composition of matter has been widely reported in peer-reviewed journals, and is typically represented by the following formula, Na8Al6Si6O24(MnO4)2-xOHx where x is from 0.1 to 0.9, and is typically 0.5. All reports on the synthesis of such Permanganate balanced Sodalite compositions have focused on the color properties of the finished pigment. None have looked at the utility of such pigments as anti-corrosive pigments. See Weller, J. Chem. Soc., Dalton Trans., 2000, 4227-4240.
The present invention affords finished compositions within the above ranges that are, by virtue of the synthetic method employed, essentially free of soluble salts. In addition, the disclosed compositions are made more suitable for use in anti-corrosive applications by adjustment of their acid stability through the employment of partial or complete silica encapsulation. Furthermore, due to the lack of trace salt contaminants, their resistance to blistering in typical coating systems is greatly improved.